If its moving through enough of an arc to actually do the job then just take it off the splined adaptor on the spindle and replace it a bit further round, as it were. If the wiper arm is all loose and floppy on the spindle then try tightening up the grub nut that holds the adaptor on the spindle. Odds are that its all worn and rocking on the end of the grub-screw- I ended up packing out the gap between the spindle and the ( new !) splined adaptor with a bit of a thin split pin as a shim - a thin nail or similar would do the job.
The older series version, where the wiper arm is held on with a nut on a splined shaft was a far better bit of engineering :-(
G.